print-stats and print-contracts do not calculate correctly

[olf42]

Both print-stats and print-contacts do not calculate the amount per month in total correctly.

$ rechnung print-contracts
1000: martha.muster@email.tld 2019-06-01 36.79€

$ rechnung print-stats
1 contracts in total
13.37€ per month

Even though, the correct amount should be 60.21€ according to this contract.

contents of _contracts/1000.yaml"

address:

• Martha Muster
• "Rosa-Luxemburg-Allee 161"
• 04161 Leipzig
  cid: '1000'
  dob: 1954-03-21
  start: 2019-06-01
  email: martha.muster@email.tld
  name: Martha Muster
  notify: false
  phone: '+491234567890'
  slug: martha_muster
  items:
  • description: A great product
    price: 13.37
    quantity: 1
  • description: Another awesome product
    price: 23.42
    quantity: 2